In an effort to cleanse the exhaust gases emanating from a diesel engine, carbon particulates occluded with hydrocarbons (soot) are collected from such exhaust by trapping and must be eliminated from the trap by periodic gasification or oxidation which requires ignition of the soot in the trap. The temperature of the exhaust gases during normal driving cycles is not high enough in passenger vehicle engine applications to ignite such soot and therefore requires some supplementary means to establish ignition and carry out oxidation. Even with truck engines, the driving cycle can create exhaust gas temperatures which are not always consistently high enough to burn off the carbon particles collected in such a trap.
It is well recognized that soot oxidation can be facilitated by means of an auxiliary fuel burner or auxiliary electric heater which functions to increase the temperature of the exhaust gases or other oxygen-carrying gas so as to bring about ignition. However, it would be desirable if such auxiliary temperature-increasing devices could be eliminated and the temperature of the normal driving cycle of the engine be relied upon to bring about ignition and carry out combustion of the collected carbon particles and occluded hydrocarbons (soot). To this end, it is desirable that the economics and reliability of carbon ignition be enhanced by some means which effectively lowers the ignition temperature of the particles.
The prior art has explored the use of various catalyst materials to reduce the ignition temperature of carbon soot (see Murphy et al, SAE Publication No. 810112, 1981 which describes carbon oxidation catalyst). In a related attempt, the prior art has learned that when a catalytic coating is applied or impregnated into the trap material, the function of regeneration (carbon oxidation) does not work as well as expected (see EPA Paper 600 7-79-232b, entitled "Assessment of Diesel Particulate Control: Direct and Catalytic Oxidation"; and a paper entitled "Catalysis of Carbon Gasification", published in Chemistry & Physics of Carbon, P. L. Webber, Jr., Editor, Vol. 4, pages 287-383, Marcel Dekker, New York, 1968).
The prior art has also turned to providing additives or injections into the fuel supply in the hopes of providing a chemical compound that would codeposit with carbon, facilitate lower ignition temperatures, and thereby provide more convenient oxidation of the carbon. Two problems are presented by such application mode: (a) the additives used heretofore have not only presented consistent problems of solubility in the fuel supply, but also are unstable over normal usage periods to maintain solubility; and (b) the inability to codeposit in a form that is effective to promote depression of the ignition temperature to a level that would accommodate exhaust temperatures reached during frequent driving cycles.
For example, in U.S. application Ser. No. 585,964, filed March 5, 1984, assigned to the assignee of this application (the disclosure of which is incorporated herein by reference), a teaching is made of a way to utilize copper and lead as additives to the fuel supply to reduce the soot oxidation temperature. The additive formulation consisted of adding 0.25 gm/gal of fuel in the form of copper napthanate and 0.5 gm/gal of fuel as lead in the form of tetraethyl lead. Although the formulation as added to the fuel supply was effective in reducing the ignition temperature of soot, it was found that the liquid additive formulation was extremely unstable in diesel fuel and required an eloborate on-board additive dispensing system to make it suitable for the vehicular applications. In addition, lead additives are toxic and pose serious problems relating to regulations for their use in diesel fuel in the United States. More importantly, reductions in ignition temperatures to levels experienced in ordinary engine operation was not achieved and the oxidation process was not necessarily sustainable when the particles were ignited.
To solve the solubility problem, U.S. Pat. No. 2,622,671 had long ago proposed that copper salts of alkanoic acids be used to achieve ignition temperature depression in connection with oil burning equipment such as oil burning locomotives, fire-up torches, etc., all using extremely large fuel burning nozzles. The disclosure of the '671 patent describes the copper salts as being of the type having a branch chain acyclic aliphatic carboxylic acids of 5-12 carbon atoms, and in which the carboxyl group is attached to a carbon atom other than the central carbon atom in the longest hydrocarbon chain. These useful copper salts of alkanoic acids were found to be suitable only with oil burners with large nozzles, but would be completely unacceptable in achieving ignition temperature depression in a vehicular particulate trap substantially removed from the burning location and where very small, intricate trap passages are involved with a relatively lower and cooler exhaust flow therethrough. Moreover, soot generation in such large oil burners occurs at a very low pressure environment (1.5 bar) and is due to the very low air/fuel ratio allowing the carbon to break down prior to combustion. The environment within a vehicular engine operation is different since the air/fuel ratios are quite large with pressures exceeding 20 bar. In fact, such air/fuel ratios in vehicles can be 80 or more while still obtaining carbon deposits. Still further, the mere use of copper salts of alkanoic acids as an additive to the fuel supply is insufficient to obtain significant ignition temperature depression of soot in a particulate trap of an automotive vehicle, principally because the additive, by itself, does not provide compounds which lay down in a sufficiently fine particle size and spacing to promote catalytic ignition at normal driving conditions.
More importantly, not all of the delineated salts in U.S. Pat. No. 2,622,671 would reduce the carbon ignition temperature sufficiently low and certainly not to a range below 700.degree. F. In fact, none of such salts would do so by itself when injected as an additive to a diesel fuel supply. Even though such salts respond to the definition of a metal octoate salt of the formula [COH]M, with which this invention is concerned, most of these salts are incapable of forming an oxide which upon heating can be finely distributed.